
def prost(t):
    chis=[]
    flag=0
    for n in range(1,t+1):
        flag = 0
        for i in range(2,n//2+1):
            if n%i==0:
                flag=1
        if flag == 0:
            chis.append(str(n))
    return (chis)
s = input()
k = 0
a = []
try:
    k=int(s)
    if (999<k<10000):
        for x in prost(k):
            if x in s:
                a.append(int(x))
        print(max(a))
    else:
        print(0)
except:
    print(0)
